Effect of stoichiometry on the evolution of thermally annealed long-range ordering in Ni–Cr alloys

Ni-based alloys, such as alloys 690 and 625, are widely used in the nuclear industry as structural components, because of their desirable mechanical properties and resistance to stress corrosion cracking. However, in some high chromium alloys, a disorder-order phase transformation near 33 at% Cr, is known to decrease ductility and fracture toughness. In this study, the ordering transformation is investigated in Ni–Cr binary model alloys to better understand the effects of composition. Model alloys with different stoichiometries (Ni/Cr = 1.8, 2.0, 2.2, 2.4) were isothermally aged up to 10,000 h at three temperatures (373 °C, 418 °C, and 475 °C) and characterized by transmission electron microscopy (TEM), microhardness, and synchrotron-based X-ray diffraction (XRD). TEM results show the evolution of the Ni₂Cr (MoPt₂-type) ordered precipitates between 3000 h and 10,000 h with corresponding size of ∼10–20 nm. Microhardness testing results show that off-stoichiometry (Ni/Cr ≠ 2.0) alloys exhibit a smaller change with ordering compared to the stoichiometric (Ni/Cr = 2.0) alloy at all temperatures. XRD quantifies ordering induced lattice contraction in the matrix structure and the size of the ordered precipitates. No BCC Cr was detected by XRD or TEM during characterization in the range of 29.83–35.66 at% Cr after 10,000 h of aging, confirming that all of the hardening can be attributed to the development of Ni₂Cr in alloys ranging from Ni/Cr of 1.8 to Ni/Cr of 2.4.